The incorporation of wind-powered generation into overall power generation strategies is continuing to increase. To improve the reliability of power systems with large-scale wind power integration, different countries have specified fault ride through (FRT) requirements for wind turbines (WT) in their respective grid codes. An example FRT requirement curve for WT (e.g., per German and Irish grid codes) is illustrated in FIG. 1. According to chart 100, a WT must stay connected when the terminal voltage remains above line 102. In addition to remaining connected to the transmission system, FIG. 2 illustrates that grid codes may put a stringent requirement on the reactive current supplied by a WT to improve the voltage security of the system. The example grid code requirements may be summarized as follows: a WT is required to stay connected during voltage dips above the specified level for the time duration set forth in chart 100, the WTs must support the grid voltage with additional reactive current during voltage dips as set forth in chart 200 (e.g., wherein, the voltage control must take place within 20 ms after the fault recognition), and the active power output of the WT must be continued immediately after the fault clearance and increased to the original value with a gradient of at least 20% of the rated power per second. These requirements are illustrated in chart 200 wherein the reactive current supplied by the WT must behave in accordance with line 202.
DFIGs are often used for grid-connected variable-speed WTs because of their ability to generate power with high-efficiency while still allowing for independent control of active and reactive power using partial capacity converters. In conventional DFIG architecture, the stator windings are directly connected to the grid and the rotor windings are connected to the grid via back-to-back connected voltage source converters (VSCs). These two VSCs can be identified as a rotor side converter (RSC) connected between the rotor and DC link, and a grid side converter GSC connected between the grid and DC link via interfacing inductors. As the stator windings are directly connected to the grid, a grid fault may cause a voltage dip at the DFIG terminals that directly affects the air-gap flux, and thus, the energy conversion process. Depending on the type of fault, the voltage dip may introduce a DC component, or a combination of a DC component and reverse rotating AC component, in the air-gap flux. These flux components induce high voltage in the rotor windings at rotational and/or double the rotational frequency. The RSC itself cannot limit these high frequency voltages due to modulation index constraint, and thus, loses its current control capabilities. Unless the proper mitigating measures are employed, the rotor currents under the grid fault condition can exceed the transient current ratings of the RSC. Grid faults may also cause severe mechanical stress on the bearings and the gear box of WECS due to torque pulsation.
Many solutions have been proposed to provide or improve the FRT capability of DFIG wind turbines. For example, during a grid fault a rotor crowbar may be used to disable the RSC and short circuit the rotor windings through resistors to limit the transient currents. A substantial drawback of this scheme is that a DFIG consumes huger reactive power during a grid fault, and shorting the DFIG in this manner may further aggravate voltage dips. Series dynamic resistors may also be employed to maintain the stator voltage or rotor currents within the limits, but may not be appropriate if the DFIG is being controlled to supply reactive power to the grid. Further, the above mentioned fault ride through techniques fail to maintain the pre-fault voltage across the stator windings, which causes undesired electrical and mechanical transients in the system.
Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications and variations thereof will be apparent to those skilled in the art.